This invention relates to hybrid power systems and, more particularly, to the control of such systems.
Cross reference is hereby made to two copending applications filed in the U.S. Patent Office and assigned to the same assignee as this application: application of J. Werth and I. Fekete entitled "Fuel Cell/Battery Hybrid System Having Battery Charge-Level Control", Ser. No. 06/537,460, filed Sept. 29, 1983, now U.S. Pat. No. 4,931,947; and application of J. Early and J. Worth entitled "Fuel Cell/Battery Control System", Ser. No. 07/296,283.
In planning for future energy needs based on an evaluation of existing and potential economic and ecological problems, a great deal of attention is being given to the development and utilization of energy efficient systems. Of particular interest are devices capable of generating electricity by consuming plentiful or renewable fuels, the utilization of which minimizes environmental pollution which is economically justified. Fuel cells and gas turbines utilizing low weight hydrocarbons or hydrogen obtained by reforming methanol or similar organic fuels represent likely candidates in this area. The fuel cell typically has a high efficiency and is largely without polluting emissions. Noteworthy also is the versatility of the fuel cell in respect to size and power level which is by adaptability to a modular design. Additionally, the fuel cell requires few moving parts, and is a quiet, reliable and comparatively maintenance free source of electric power.
The above factors have made the fuel cell a likely candidate for use in specialized vehicle transportation where needs are well defined and long-standing. Unfortunately, fuel cells do not generally operate as well as desired under conditions in which substantial variation in the load or surge demand are encountered. Fuel cell components can be damaged by surge demands due to overheating, or the surge demand may not be met at all. To overcome this problem, attempts have been made to utilize fuel cell stacks as primary energy sources in a so called "hybrid system", the purpose of which is to allow use of the major energy source (fuel cell) in a near optimum design by supplying only the average load requirements directly therefrom.
Such a system includes a storage source, such as a battery, to supply the transient load increases and which can be recharged when the load drops below average. Thus, the load variation experienced by the primary energy source or fuel cell is reduced in frequency and magnitude. This allows for design and operation of the fuel cell to be within a band representing the average load rather than designing for peak load with dominant operation under peak load conditions.
Reduced emissions and increased efficiency are the expected benefits for operating the hybrid system. A prior art type of hybrid power device is disclosed in an article by J. B. O'Sullivan et al "Hybrid Power Source For Material Handling Equipment", IECEC 1975 Record, pp 229-236. In the system described by O'Sullivan, four fuel cell stacks are in parallel with a battery sub-system. A controller controls the fuel cell and battery sub-systems to limit the total current from the fuel cell to maintain the fuel cell's life and to limit the fuel cell sub-system voltage as protection for the batteries. At low currents, the fuel cell sub-system voltage rises rapidly and fully charged batteries would be drive into strong gasing if not protected. The O'Sullivan controller operates to vary a series resistance between the fuel cell stack and the batteries, resulting in a variable voltage drop to maintain the voltage constant at the battery at low currents and maintain the voltage constant at the fuel cell at high currents.
Although the system described in O'Sullivan provides an approach to control of the system, the degree of control may not be optimal since significient resistive losses occur during operation. Further, O'Sullivan does not specify how the sharing of load current demand is provided between the fuel cell stack and the batteries. Therefore, it appears that the O'Sullivan system does not provide a technique in which the peak surge energy demands in a hybrid system are compared to the average load requirements, and appropriate adjustments are made in the system, so that either the batteries alone or the fuel cell and the battery are utilized in some combination to meet the load demand in a most efficient manner.
In addition to the O'Sullivan article, the following U.S. patents are of general interest in the fuel cell area: U.S. Pat. Nos. 4,000,003; 3,883,368; 3,753,780; 3,546,020; and 3,473,337, and are mentioned herein as a matter of general background.
Other fuel cell/battery hybrid systems of interest are disclosed in "Fuel Cell Systems for Vehicular Applications", SAE Technical Paper Series, 800059, by Lynn, McCormick, Bobbett and Derouin, Feb. 25-29, 1980; "A Fuel Cell-Battery Power Source for Electric Vehicles", The Fifth International Electric Vehicle Symposium, 782407(E), by Dowgiallo, Oct. 2-5, 1978; UK Patent Application GB No. 2 084 387 A, published on Apr. 7, 1982; "An Assessment of the Status of Fuel Cell/Battery Vehicle Power Systems", by Escher and Foster, February, 1980; Fuel-Cell-Powered Golf Cart Report CONF-800523-1 of the Third International Electric Vehicle Exposition and Conference, St. Louis, Mo., by Bobbett, McCormick, Lynn, Kerwin and Derouin, May, 1980.